1. Field of the Invention
Aspects of the present invention relate to an electrode for a fuel cell and a fuel cell including the same, and more particularly, to an electrode for a fuel cell that has excellent ion conductivity and stability even at high temperatures because of the excellent features imparted by phosphoric acid loaded in the electrode, as well as a fuel cell including the same.
2. Description of the Related Art
Fuel cells are categorized into PEMFC (proton exchange membrane fuel cells), PAFC (phosphoric acid fuel cells), MCFC (molten carbonate fuel cells) and SOFC (solid oxide fuel cells), etc.
PEMFC fuel cells are anticipated to have wide use, for example, in standing devices, automobiles and portable devices, because of high output density, quick rise to operating voltage and high efficiency, etc. Research is underway to raise the operating temperature of existing PEMFC's from the current 80° C. to above 100° C. because a high temperature system has advantages as follows: (1) the electrochemical reaction at both electrodes becomes faster, (2) water management becomes easier, (3) the cooling system becomes simple because of the large difference in temperatures between a fuel cell stack and a refrigerant, (4) it is easy to recover waste heat, and (5) the fuel processor can be simplified because of the decrease in carbon monoxide (CO) poisoning.
However, the fundamental obstacle to a high temperature system is that a polymer electrolyte membrane is urgently required that has stability at a high temperature of about 150° C. while maintaining its proton conducting property. NAFION® (DuPont Corporation) is used as an electrolyte membrane in existing low temperature PEMFCs because NAFION® shows high ion conductivity in the water-absorbed state. NAFION® has also been used as an ion conductor and binder in electrodes because of its high conductivity. However, NAFION® is expensive, and water cannot be absorbed on NAFION® in the liquid state at a temperature of 150° C. so that the ion conductivity of NAFION® drops drastically. Accordingly, NAFION® is not attractive as a binder since it cannot serve as a proton conductor. Accordingly, for an electrode in a high temperature fuel cell system, a polymer material is required that can serve as a stable binder even for long operating times, and can enhance the electrode performance because of its inherent properties.
Currently, PTFE (polytetrafluoroethylene) or PVdF (polyvinylidenefluoride) are mainly used as binders in high temperature PEMFCs. In high temperature PEMFCs, liquid phosphoric acid is used as a proton conductor, and thus the performance of the electrode is greatly affected by the amount and distribution of phosphoric acid absorbed into the electrode. However, PTFE and PVdF are hydrophobic. Since a hydrophobic material used as a binder in an electrode serves as gas pathway, but also has the property of preventing the absorption of phosphoric acid, electrodes using a hydrophobic material have inferior performance and take too much time to reach maximum performance. Accordingly, a hydrophilic polymer material that is stable at high temperature is required as a binder